Where a primary source presents material from a first-hand witness to a phenomenon, and a secondary source provides commentary, analysis and criticism of primary sources, a tertiary source is a selection and compilation of primary and secondary sources. While the distinction between primary source and secondary source is essential in historiography, the distinction between these sources of evidence and tertiary sources is more peripheral.
Typical instances of tertiary sources are bibliographies, library catalogs, directories, reading lists and survey articles. Encyclopaedias and textbooks are examples of materials that typically embrace both secondary and tertiary sources, presenting on the one hand commentary and analysis, while on the other attempting to provide a synoptic overview of the material available on the topic. For instance, the long articles of the Encyclopędia Britannica certainly constitute the kind of analytical material characteristic of secondary sources, whilst they also attempt to provide the kind of comprehensive coverage associated with tertiary sources.
It may be interesting to note that Wikipedia may be considered a tertiary source, and that this article itself is a meta-tertiary source.
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